1. Field of the Invention
This invention generally concerns protection from laser radiation for optical equipment and human eyes and anything else requiring such protection.
2. Description of Related Art
Laser light is a high intensity monochromatic radiation having extremely high coherence. An increasingly large number of applications based on lasers are currently available. Most applications in the consumer sector incorporate low intensity lasers such as compact discs, DVDs and other optical devices, while high intensity lasers largely remain in the research, medical, defense, industrial, nuclear and astronomy sectors. Low energy lasers are commonly used in law enforcement and warfare for target illumination.
Development of both low and high power laser systems has reached the maturity where such systems are economically and technically feasible. The main requirement for protection against laser damage of optical receptors, especially in the case of high sensitivity devices, is high transparency under low intensity and ambient conditions combined with opacity under high intensity radiation.
Laser eye protection (LEP) incorporates cutting edge technologies (reflective coatings and advanced absorbing dyes) to protect against lasers at wavelengths in the near-infrared (NIR) and visible portions of the electromagnetic spectrum. However, these technologies produce filters that always block the light for which they are designed, whether under laser illumination or not. When these filters block visible light they have a negative impact on visibility for the person using such eyewear, including even some filters intended only for NIR protection. These negative effects increase proportionally with the number of visible wavelengths blocked until the filter becomes opaque. Active filters, i.e., those blocking light only when illuminated with a laser, are a conceptual solution to this problem; however, they currently do not respond fast enough to protect against pulsed laser systems in nanosecond time domains. Pulsed emissions can be created at numerous wavelengths, and the high peak power in very short pulses can cause retinal injury at average energy outputs that would not be injurious for a continuous wave emitter.
Optical power limiters (OPLs) are materials and devices designed to allow normal transmission of light at low intensities and limited transmission of light at higher intensities. They function so as to form an optical barrier as a direct response to excessive intensity of light, thus providing a promising technology currently under development for protecting against pulsed lasers. OPLs are nonlinear optical materials and devices designed to allow normal transmission of light at low intensities and limited transmission of light at higher intensities [e.g., Spangler, C. W. et al., Proc. MRS Symp., 479, 59 (1997)].
There are various important considerations that go into the design of an OPL device. The threat of very short intense laser light pulses requires a device with extremely rapid response to changing light intensity. Fast response times favor materials-based devices over mechanical ones. The material must be able to withstand the impact of prolonged exposure to high intensity light, as well as to allow for continuous transparency in regions outside the path of the high intensity light.
The problem with current OPL technology for LEP is that the threshold energy for nonlinear behavior, i.e. OPL activation energy threshold is much higher than the threshold for retinal injury. Because of this fact, current OPL materials require a lens system to collect and focus the incident energy on the OPL in order to create a nonlinear behavior at incident energies relevant to retinal protection. This requirement translates into heavy and bulky LEP devices. An OPL that doesn't require a focal plane would result in LEP that is significantly smaller and lighter than current LEP devices allow. Advantages of such devices for personal protection are obvious. Consequently, for over a decade, several research groups have been attempting to develop novel OPL materials based on nonlinear optical (NLO) dyes. [See, for example, Spangler, C. W. et al., Proc. MRS Symp., 479, 59-67 (1997); Albota, M. et al., Science 281, 1653-1656 (1998); Hernandez, F. E. et al., Opt. Lett. 25, 1180-1182 (2000); Drobizhev, M. et al., Opt. Lett. 26, 1081-1083 (2001); He, G. S. et al., Appl. Phys. Lett. 67, 2433-2435 (1995); and Perry, J. W. et al., Opt. Lett. 19, 625-627 (1994).]
In order to be useful for practical applications, however, an OPL material must fulfill all of the following requirements:                1. It must have a fast response time.        2. It should operate over a broad wavelength range.        3. The on-off cycle must be extremely fast, ideally following the cycle frequency of the laser pulse it is responding to.        4. It must have a low threshold activation energy for OPL onset.        
OPL devices rely on one or more nonlinear optical mechanisms, which include: (1) Reverse Saturable Absorption (RSA); (2) Two-Photon Absorption (TPA); (3) Multi-Photon Absorption (MPA); (4) nonlinear refraction; (5) induced scattering; and (6) photorefraction. A number of these processes have been extensively studied for OPL applications. [See, for example, Van Stryland, E. W. et al., Nonlinear Optics of Organic Molecules and Polymers, (Eds.) Nalwa, H. S.; Miyata, S., CRC Press, New York, pp 841-860 (1997); Crane, R.; Lewis, K.; Van Stryland, E. W.; Khoshnevisan, M. (Eds.), Materials for Optical Limiting-Proceedings of the Materials Research Society Symp., MRS, Pittsburgh, 374, pp. 341-347 (1995); Lawson, C. M. (Ed.), Nonlinear Optical Liquids and Power Limiters-Proceedings of the SPIE, SPIE, Bellingham, Wash., 3146, pp 54-60 (1997); Xia, T. et al., Appl. Opt. 36, 4110-4122 (1997); and Zhou, G. et al., Appl. Opt. 41, 1120-1123 (2002).] To date, however, there is no one OPL material available which, taken individually, can provide ideal and smooth attenuation of an output beam. Therefore, the design and development of radically novel types of materials for OPL are urgently required.
In this regard, some attempts were made with combinations of NLO materials in cascading geometries, such as multi-plate or tandem cells [see Miles, P. A. Appl. Opt. 33, 6965-6979 (1994)] and the use of two intermediate focal planes in a sighting system [see Van Stryland, E. W. el al., Nonlinear Opt. 27, 181-192 (2001)].
The human eye is a very sensitive optical sensor with a very low damage threshold for the retina (˜0.5 μJ into the pupil of the eye or 40 μJ/cm2). This imposes stringent demands on materials for laser protection. Existing nonlinear optical materials can respond to such low energies only when the light is tightly focused—this is achieved most easily in an optical system which provides focal planes at which the nonlinear material can be positioned.
Clearly, it would be desirable to have such materials available.